High lead levels may permanently lower IQ

Exposure to lead has a lasting effect on a child’s intelligence, according
to a long-term study of almost 500 children living near a lead smelter at
Port Pirie, South Australia. The study suggests that lead in the environment
may permanently impair IQ.

The research challenges a belief among some researchers that lead merely
delays intellectual development. ‘The thought before was that maybe the
children might catch up as they got older,’ says Peter Baghurst, of the
CSIRO division of human nutrition in Adelaide. ‘But this does not seem to
be the case. At age seven they are leaving it rather late.

The South Australian study, a joint project between the University of
Adelaide, the CSIRO and several medical institutions in Adelaide, is the
first to examine the intellectual development of children exposed to lead
in the environment from birth to school age. The results show that exposure
to lead produces a ‘developmental deficit’, says Baghurst.

Compared with the effect of genetic or social factors such as parental
IQ and education, the impact of lead on the child is small, but still significant.
The results will be published by the CSIRO next week in its quarterly environmental
magazine ECOS.

Advertisement

In 1989 the Australian researchers reported that postnatal exposure
to lead was associated with impaired mental development at two and four
years of age. The latest findings make the association more conclusive,
says Baghurst.

In the latest study, those children who had the highest concentrations
of lead in their blood were on average less intelligent than those who had
the lowest. According to Baghurst, the difference between those with the
highest concentrations and those with the lowest is equivalent to the difference
between a child of average IQ and one regarded as slightly retarded. After
correcting for other factors, such as parental education, family income,
maternal age and quality of the home environment, the difference was smaller
but still significant, the researchers say.

When the children were three years old, the researchers graded them
according to the concentration of lead in their blood. At seven, when they
measured IQ, they found a 5 per cent difference in IQ between those children
who at three years old had 300 micrograms of lead per litre of blood and
those with 100 mu g/l. The Australian safety limit for lead is 250 mu g/l.
In the US, the Centers for Disease Control recently lowered the safety limit
from 250 to 100 mu g/l.

The seven-year-olds were given standard IQ tests to measure cognition
and mental function, including their ability to assess spatial relationships
and manipulate abstract symbols. The researchers claimed they were unable
to observe a threshold level below which exposure to lead had no effect
on IQ, and they suspected that their estimates were conservative.

Baghurst says that lead may not have as great an effect on a child’s
IQ as other factors such as parental IQ, but its impact could be widespread.
Many thousands of children in urban and industrial areas may be suffering
mild intellectual impairment because of exposure to lead dust from mining
and smelting operations, old lead-based house paint and leaded petrol.

The New South Wales Health Department recently reported that in Broken
Hill, one of Australia’s biggest mining communities, more than one in four
young children had levels of lead in their blood over the Australian safety
limit.